This invention relates to a semiconductor light emitting device and its manufacturing method. More particularly, the invention relates to a light emitting device having a stacked nitride compound semiconductor layer of GaN, InGaN, GaAlN, or the like, which remarkably reduces the operation voltage of the device, increases the luminance, and improves the reliability, and a manufacturing method for manufacturing such a device.
Light emitting devices for wavelength bands from ultraviolet to green or blue are being brought into practice by using nitride compound semiconductors represented by gallium nitride.
In this application, “nitride compound semiconductors” involve III-V compound semiconductors expressed as BxInyAlzGa(1−x−y−z)N (0≦x≦1, 0≦y≦1, 0≦z≦1), and further involve mixed crystals containing phosphorus (P), arsenic (As), etc., in addition to nitrogen (N) as group V elements.
It is getting possible to realize emission of light with a high intensity which has been difficult heretofore, such as ultraviolet light, blue light and green light, for example, by making light emitting devices like light emitting diodes (LED) and semiconductor lasers using nitride compound semiconductor. Moreover, because of their crystal growth temperatures being high, nitride compound semiconductors are stable materials even under high temperatures, and their use to electronic devices is hopefully expected.
A review is made on LED as an example of semiconductor light emitting devices using nitride compound semiconductors.
FIG. 14 is a conceptional diagram showing a cross-sectional structure of a conventional nitride compound semiconductor LED. The conventional LED is made of a GaN buffer (not shown), n-type GaN layer 102, InGaN light emitting layer 103 and p-type GaN layer 104 which are epitaxially grown on a sapphire substrate 101 sequentially. The InGaN light emitting layer 103 and the p-type GaN layer 104 are partly removed by etching to expose the n-type GaN layer 102. Formed on the p-type GaN layer 104 is a p-side transparent electrode 113. Further stacked on a part of the p-side electrode 113 are an insulating film 107 for blocking current and a p-side bonding electrode 106. Formed on the n-type GaN layer 102 is an n-side electrode 105.
In this structure, a current injected through the p-side electrode 106 is spread out by the transparent electrode 113 having a good conductivity and injected from the p-type GaN layer 104 into the InGaN layer 103 to emit light there. The emitted light is led out outside the chip through the transparent electrode 113.
However, conventional nitride compound semiconductor light emitting devices as shown in FIG. 14 involved problems, namely, high contact resistance at electrode portions and insufficient external quantum efficiency of light.
That is, since GaN has a band gap as wide as 3.4 eV, it is difficult to ensure its ohmic contact with electrodes. This invites the problems that the contact resistance increases in electrode portions, operation voltage of the device increases, and a large amount of heat is generated.
Moreover, refractive index of GaN is as large as 2.67, and its critical angle of refraction is as very small as 21.9 degrees. That is, when viewed from the normal of the surface from which the light exits, light which enters with a larger angle than the critical angle of refraction cannot be led outside the LED chip. Even when an AR (anti-reflection) film is coated on the surface of the chip, this critical angle does not change. Therefore, it has been difficult to obtain a larger emission power by improving the external quantum efficiency.